The invention relates generally to integrated circuits and more particularly to a double sided lower electrode capacitor for use in an integrated circuit.
Capacitors are used in a wide variety of semiconductor circuits. Capacitors are of special concern in DRAM (dynamic random access memory) memory circuits; therefore, the invention will be discussed in connection with DRAM memory circuits. However, the invention has broader applicability and is not limited to DRAM memory circuits. It may be used in any other type of memory circuit, such as an SRAM (static random access memory), as well as in any other circuit in which capacitors are used.
DRAM memory circuits are manufactured by replicating millions of identical circuit elements, known as DRAM cells, on a single semiconductor wafer. A DRAM cell is an addressable location that can store one bit (binary digit) of data. In its most common form, a DRAM cell consists of two circuit components: a storage capacitor and an access field effect transistor.
FIG. 1 illustrates a portion of a DRAM memory circuit containing two neighboring DRAM cells 10. For each cell, one side of the storage capacitor 14 is connected to a reference voltage, which is typically one half of the internal operating voltage (the voltage corresponding to a logical xe2x80x9c1xe2x80x9d signal) of the circuit. The other side of the storage capacitor 14 is connected to the drain of the access field effect transistor 12. The gate of the access field effect transistor 12 is connected to a signal referred to as the word line 18. The source of the field effect transistor 12 is connected to a signal referred to as the bit line 16. With the circuit connected in this manner, it is apparent that the word line controls access to the storage capacitor 14 by allowing or preventing the signal (a logic xe2x80x9c0xe2x80x9d or a logic xe2x80x9c1xe2x80x9d) on the bit line 16 to be written to or read from the storage capacitor 14.
The manufacturing of a DRAM cell includes the fabrication of a transistor, a capacitor, and three contacts: one each to the bit line, the word line, and the reference voltage. DRAM manufacturing is a highly competitive business. There is continuous pressure to decrease the size of individual cells and increase memory cell density to allow more memory to be squeezed onto a single memory chip. However, it is necessary to maintain a sufficiently high storage capacitance to maintain a charge at the refresh rates currently in use even as cell size continues to shrink. This requirement has led DRAM manufacturers to turn to three dimensional capacitor designs, including trench and stacked capacitors. Stacked capacitors are capacitors which are stacked, or placed, over the access transistor in a semiconductor device. In contrast, trench capacitors are formed in the wafer substrate beneath the transistor. For reasons including ease of fabrication and increased capacitance, most manufacturers of DRAMs larger than 4 Megabits use stacked capacitors. Therefore, the invention will be discussed in connection with stacked capacitors but should not be understood to be limited thereto. For example, use of the invention in trench or planar capacitors is also possible.
One widely used type of stacked capacitor is known as a container capacitor. Known container capacitors are in the shape of an upstanding tube (cylinder) having an oval or circular cross section. FIG. 2 illustrates a top view of a portion of a DRAM memory circuit from which the upper layers have been removed to reveal container capacitors 14 arranged around a bit line contact 16. Six container capacitors 14 are shown in FIG. 2, each of which has been labeled with separate reference designations A to F. Recall from FIG. 1 that the bit lines of neighboring DRAM cells are electrically connected. To increase density, bit line contacts are shared by neighboring DRAM cells. In FIG. 2, the bit line contact 16 is shared by DRAM cells corresponding to container capacitors A and B. The wall of each tube consists of two plates of conductive material such as doped polycrystalline silicon (referred to herein as polysilicon or poly) separated by a dielectric. A preferred dielectric is tantalum pentoxide (Ta2O5) . The bottom end of the tube is closed, with the outer wall in contact with either the drain of the access transistor or a plug which itself is in contact with the drain. The other end of the tube is open (the tube is filled with an insulative material later in the fabrication process). The sidewall and closed end of the tube form a container; hence the name xe2x80x9ccontainer capacitor.xe2x80x9d Although the invention will be further discussed in connection with stacked container capacitors, the invention should not be understood to be limited thereto.
The electrodes in a DRAM cell capacitor must be conductive, and must protect the dielectric film from interaction with interlayer dielectrics (e.g., BPSG) and from the harsh thermal processing encountered in subsequent steps of DRAM process flow. For example, Ta2O5 dielectrics may be used for high density DRAMs, such as 64 Mbit and 256 Mbit DRAMs, because chemical vapor deposition (CVD) of Ta2O5 provides a high dielectric constant (about 20-25) and good step coverage. However, when rapid thermal processed nitride (RTN) is formed over a layer of hemispherical grain polysilicon (HSG) to serve as an HSG barrier layer to prevent oxidation of HSG during subsequent Ta2O5 deposition, there is a capacitance loss due to the RTN layer on the capacitor electrode. The effective dielectric constant for an RTN/Ta2O5 stack capacitor is about 10-12.
Several methods have been attempted to increase capacitance, including depositing HSG inside a container capacitor together with a smooth polysilicon deposited on the outside of the container, depositing a smooth metal on both the inside and outside of the capacitor, and depositing a double sided HSG. The present invention has advantages over the previous methods in that the HSG inside with smooth polysilicon outside capacitors, and the smooth metal capacitors, have lower capacitance and do not achieve the superior results of the present invention. Regarding the double sided HSG capacitor, the present invention also overcomes the difficulties involved in using HSG on the outside of a capacitor plate, and the resulting in short circuits between containers and defects in the capacitor.
As memory cell density continues to increase, there is needed a capacitor that has an increased effective capacitance per cell. The present invention provides a fabrication process and capacitor structure that achieves high storage capacitance without increasing the frequency of capacitor defects or the size of the capacitor.
The present invention provides a double sided electrode capacitor formed of a metal electrode that preferably has an additional conductive layer, preferably HSG, on one side, together with a silica nitride coating dielectric, and a dielectric material such as Ta2O5 on another side, as shown, for example, in FIG. 3.